2 Answers
2

Generally speaking, treat the sukkah as your dining room. If you had a massive leak in your dining room roof, you would eat elsewhere (potentially even going to a neighbor's house).

There is a stricter standard for eating one piece of bread on the first night of sukkos, but otherwise:

If it rains enough that the rain is ruining your soup

If it's severely hot, cold, or buggy in ways that you can't mitigate (e.g. cold, just wear a coat) to the point that it's seriously ruining your meal. (In the past rabbis wrote about being so cold that the fat in the food started to congeal, today we're not quite as tough.)

If there are too many people, "crowded" per se isn't a way out. However, we would apply a priority scheme for who should be in the sukkah, starting with those who must and ending with those for whom it's meritorious. The only ones who MUST be are adult (age 13+) males, eating bread or possibly other grain products. So the Atkins people should stay inside if there's really no room in the sukkah.

Lechatchila, one should build a sukkah that accomodates all. But what if a person must use a community sukkah and has no say about asking exempties to leave. Is there a source that crowded, noisy or other similar issues is not a p'tur?
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YDKOct 6 '11 at 14:06

I have heard from what I considered reasonably Orthodox rabbiem that as in the answer from Shalom that you should treat it like your dining room, but taken to a slightly different level. If conditions were such that it was uncomfortable to eat in your dining room, you would not. And thus it applies to the Succah.

Ordinarily I eat in the dining room. One Shabbat when the AC was on in the bedroom and not the dining room, my wife & I took our dinner in the bedroom. If the sukkah is that hot, am I exempt?
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Ze'ev FelsenOct 5 '12 at 2:00